1. Field of the invention
The present invention generally relates to an atmosphere furnace such as a heating furnace, gas furnace and low pressure furnace and more particularly to an atmosphere furnace suitable as a reflow furnace for melting by heating a cream solder for soldering by melting cream solder.
2. Description of the related art
As shown in FIG. 6, a conventional tunnel-like furnace has a housing 31 divided into a plurality of chambers 33 by partitions 32. Each chamber 33 is provided with a heater 34 and a fan 35 for chamberwise temperature control. Through this furnace housing 31 there is provided a conveying means 36 for a work 37. It is so arranged that the work 37 is heated according to a given temperature profile by conveying the work 37 to pass through the individual chambers 33 in the furnace housing 31 successively to thereby melt a cream solder.
In the aforementioned reflow furnace, however, the atmosphere was bound to leak through an opening for work made in the partition 32 between the adjacent chambers 33 into the adjacent chamber or to the outside space, hence it was difficult to accurately control the temperature of each chamber 33 and the thermal efficiency was very poor due to a large heat loss.
Also, in order to do melting, it is preferred to first preheat the work from the normal temperature to approximately 150.degree. C., then heat to not less than 200.degree. C. and keep that temperature for a given length of time and then lower the temperature again to 150.degree. C. or so and after keeping at this temperature for some time to gradually cool to the normal temperature. With the aforementioned reflow furnace, however, the temperature profile is as shown in FIG. 5c, and since the maximum temperature is approximately 250.degree. C. in order to keep a temperature of approximately 200.degree. C. for a predetermined length of time, the furnace of this type is good for only works of high heat resistance.
In order to solve these problems it is conceivable to use an atmosphere furnace having therein a plurality of divided furnace units closed from the outside space and individually temperature-controllable and, moreover, a work transfer means which allows transfer of works between adjacent divided furnace units with the atmosphere contained in each unit separate from that in adjacent units as well as shut from the outside space so that each work can be transferred from one divided furnace unit to an adjacent one in order. With this type of atmosphere furnace a high precision temperature control is feasible in the absence of leakage of atmosphere from each divided furnace unit. Moreover the thermal efficiency is high and the temperature in each divided furnace unit can be kept at a given level uniformly, hence a temperature profile as shown in FIG. 5b is obtainable, hence the work can be kept at no less than the predetermined temperature for the predetermined length of time without any risk of the work being exposed to a temperature substantially higher than that. Meanwhile, however, there is a problem that, as the work moves from the outside space to a divided furnace unit or from a divided furnace unit to an adjacent one, it is subjected to thermal stress, this causing damage or trouble to the work.